How is a first-degree AV block characterized on an EKG?

A first-degree AV block is characterized on an EKG by a constantly prolonged PR interval that exceeds 0.2 seconds. This means that the time from the onset of the P wave to the onset of the QRS complex is longer than the normal 0.12 to 0.20 seconds. In this condition, each electrical impulse from the atria is delayed as it passes through the AV node but is eventually conducted to the ventricles. The consistent nature of the prolonged PR interval is what distinguishes this particular block from others, such as second-degree or third-degree AV block, where the PR interval may vary or be completely absent.

The other choices do not accurately describe a first-degree AV block. An irregular PR interval would suggest a different type of block or arrhythmia altogether, while the presence of multiple P waves could indicate other conduction issues, such as atrial flutter or multifocal atrial rhythm, and a flatline appearance might suggest asystole or severe bradycardia, which are not characteristics of first-degree AV block.